1. Technical Field
This invention pertains to the field of electronic image generation and, more particularly, to scanning apparatus for producing electronic picture signals from a color original and to signal processing techniques for use therewith.
2. Background Art
Although generally useful in the electronic imaging art, this invention has special application to a linear array film scanner used in a telecine machine for producing a television signal from a motion picture film. A linear array film scanner typically uses a light-sensitive linear charge-coupled device (CCD), which provides a serial output representing a line of a television raster. For color television, a film scanner usually includes an assembly of three separate CCD arrays, one each for red, green and blue. The film is driven at a uniform rate between the linear array assembly and a light source in a direction perpendicular to the linear dimension of the sensor arrays, and a beam splitting optical system images an illuminated section of film on each CCD array. The film motion provides the vertical (frame) scan and the linear cycling of the CCD arrays provides the horizontal (line) scan. Such a scanner is described in U.S. Pat. No. 4,205,337.
Instead of using separate linear devices, U.S. Pat. No. 4,736,251 discloses three CCD line sensors formed on a single solid state substrate. Because different illuminated sections of the film are imaged on the respective line sensors, the signals output from the sensors must be corrected by using shift registers or memory to obtain identical timing in the vertical direction. The '251 patent also discloses a CCD array incorporating four line sensors--three color sensors and a fourth line sensor for deriving a luminance signal. According to the patent disclosure, it is then possible to obtain a luminance signal having a higher resolution than that composed from the color signals, thereby allowing the capacity of memories for storing the color signals to be smaller than that for storing the luminance signal.
The extension of linear array techniques directly to high definition television, which requires more lines of much higher resolution than in conventional television, is difficult for several reasons. To begin with, existing arrays will not work fast enough to read out at the data rates required for high definition scanning. For example, a high definition frame of 1920 pixels/line for 1035 lines/frame requires a very high output data rate of at least 60 mHz--partly due to higher resolution in the horizontal (line) scanning direction and partly due to increased line scans in the vertical (frame) scanning direction. (Indeed, the data rate of 60 mHz would only be adequate if the full frame height could be used to provide data. In practice, because the high definition aspect ratio is 16:9 whereas the conventional motion picture film aspect ratio is 4:3, only about 62% of the film frame height is used. As a consequence, a much higher data rate of about 100 mHz or more is needed.) This problem can be addressed, albeit with additional complexity, by interleaving the output of the photosites to two or more output registers to lower the output clock frequency for a given data rate (which is useful in the line scanning direction, see U.S. Pat. No. 4,330,793) or by simultaneously scanning paired line sensors oriented to adjacent lines to double the number of lines for a given line rate (which is useful in the frame scanning direction, see published U.K. Patent Application GB 2191061).
More importantly, considering the very short linear scanning times involved in reaching the required resolution, the sensitivity of existing devices is not high enough to provide good signal-to-noise performance. Providing a plurality of output registers or paired sensors is a way of accommodating the shorter line times and higher data rates with a reachable clock frequency; by themselves, such techniques do nothing to increase sensitivity of the sensors. Thus a high definition film scanner operating linear arrays according to known techniques is plagued with a complex read-out architecture yielding a relatively inadequate signal-to-noise performance.